Environmental concerns have resulted in increasingly strict regulation of engine emissions by governmental agencies. For example, reduction of nitrogen-oxygen compounds (NOx) in exhaust emissions from internal combustion engines has become increasingly important and current indications are that this trend will continue.
Future emission levels of diesel engines will have to be reduced in order to meet Environmental Protection Agency (EPA) regulated levels. For example, proposed Ultra-Low Emissions Vehicle (ULEV) emission levels for light-duty vehicles up to model year 2004 are 0.2 gm/mi NOx and 0.08 gm/mi particulate matter (PM). Beginning with the 2004 model year, all light-duty Low Emission Vehicles (LEVs) and Ultra-Low Emission Vehicles (ULEVs) in California have to meet a 0.05 gm/mi NOx standard to be phased in over a three year period. In addition to the NOx standard, a full useful life PM standard of 0.01 gm/mi also have to be met. The EPA has also proposed tighter regulations for off-road diesel engines, requiring them to emit 90% less particulate matter and nitrogen oxides, by 2014 than they do today.
Traditional methods of in-cylinder emission reduction techniques such as exhaust gas recirculation (EGR) and injection rate shaping, by themselves will not be able to achieve these low emission levels required by the standards. After-treatment technologies will have to be used, and will have to be further developed in order to meet the future low emission requirements of the diesel engine.
A promising after-treatment technology designed to meet future NOx emission standards is Selective Catalytic Reduction (SCR) catalysts which have the potential to greatly reduce NOx emissions from internal combustion engines. Under some operating conditions SCR catalysts can reduce the level of NOx emitted from an internal combustion engine by as much as 60-90%. In SCR catalytic reduction systems, a reductant, such as urea, is introduced into the exhaust stream upstream of the catalyst chamber to react with the NOx on the surface of the precious metal catalyst to convert NOx into nitrogen and water vapor which is then released into the atmosphere. An accurate measurement of the exhaust flow is required in order to dose the correct amount of urea for obtaining the maximum NOx conversion efficiency with minimal NH3 slip at the tailpipe of the system.